The invention relates to a measuring device for measuring the tensile stress of an optic fiber or a corresponding wire. The invention also relates to a method for measuring the tensile stress of an optic fiber or corresponding wire.
An optic fiber is a thin glass fiber whose diameter is about 150 micrometers. Optic fibers are used to replace copper conductors in telecommunications paraphernalia, since the data transmission capacity of optic fibers are much higher than that of copper conductors. Furthermore, there is no shortage of the raw material of the optic fiber, i.e. glass, as is the case with copper.
Optic fiber is prepared in a so-called draw tower, which is a large-sized tower with a height of about 6 m to about 7 m. At the upper end of the tower, molten glass is drawn downwards into a thin fiber. The hot fiber cools as it progresses downwards through the tower. At the lower end of the tower, the fiber may be coated with a desired polymer in a special coating vat. After the coating step, the fiber passes over a tension meter and is transported onto a special roller. The fiber is wound into a roll on the roller.
It is known that bare optic fiber, i.e. an uncoated optic fiber, cannot bend since it breaks apart. However, after the fiber is coated by the polymer coating, the fiber is very resistant to bending.
In conventional draw towers, the draw rate is generally about 2 m/s to about 4 m/s. The draw rate of the fiber and the thickness of the fiber are adjusted with by means of the tension meter. It is not possible to measure the tension stress of the fiber before the fiber is coated since a bare fiber cannot be contacted. However, a possible tension measurement taken after the fiber is coated is not useful as it would provide an incorrect result of the tension of the bare, uncoated fiber.
As known in the art, it is desirable to increase the draw rate of the fiber in the draw tower. It is preferable from the point of view of the process control that the tension of the fiber should be measured without contacting the fiber. A current fiber tension meter used in draw towers is a spring-loaded roller over which an uncoated optic fiber advances. The rate of the loading of the roller indicates the fiber tension of the optic fiber. However, such a roller that contacts the bare, uncoated fiber easily breaks the bare fiber and is only used at very slow draw speeds when the drawing machine starts operation.
With respect to the prior art, an electro-optic meter may be mentioned as one device to measure tensile stress. In this type of instrument the tensile stress of the fiber is measured on the basis of the double refraction of light. The glass-fiber material in the meter is often quartz in which a double refraction of light occurs which is comparative to the tension in the fiber. A problem associated with this type of meter is the fact that the light has to be accurately focused on a movable glass-fiber of a strength of about 150 micrometers. After the light is focused, the double refraction is measured.
In Finnish Patent No. FI 79410 and U.S. Pat. No. 4,833,928, a method is described in which the tension of a movable thin film or membrane is measured by measuring the speed of the membrane wave acousto-optically. A loudspeaker is brought near the membrane, by means of which a mechanical surge is brought about onto the membrane. The mechanical surge advances in the direction of the membrane tension. The advancing speed of the surge is measured by optically measuring the vibration of the membrane at two different points on the membrane. When the wave speed and the basis weight of the membrane are known, the tension may be calculated. However, such a method cannot be used for measuring the tensile stress of a thin transparent glass-fiber. The reason for this is because the two physical wave movements are described by two different equations, since the phenomena are different phenomena, i.e. the wave movement of a thin wire is physically different as compared to the wave movement of a planar membrane.
In U.S. Pat. No. 5,079,433, a device is described which measures the tension of a fiber by causing a standing wave movement by means of pressurized air and then measuring the basic frequency of the standing wave. On the basis of these measurements, the fiber tension is calculated. In the device described in that patent, very complex calculative arrangements have to be performed, such as doubling the frequency etc., in order to calculate the fiber tension. The standing wave movement and its frequencies also depend on the fiber length to be measured or the fixing points of the fiber. These measurements are determined by the dimensions of the drawing machine used. This results in a significant disadvantage if the fiber length or fixing points of the fiber are unknown or indeterminable.
In European Patent publication EP 0 226 393, a measuring method is described wherein the fiber tension is measured by electro-optical means without contacting the optic fiber. The vibration frequency of the fiber is indicated by a so-called CCD-camera whereby a fiber shadow is caused to fall onto a surface of the CCD-camera. Also in this measuring method, a standing wave is produced along the entire fiber length, and a so-called basic frequency is searched for by means of the Fourier analysis. From these measurements, the speed of the wave movement, and thereafter the fiber tension, is calculated.